Detection of the [O I] 63 $\hat{I}¼$m emission line from the $z=6.04$ quasar J2054-0005

Abstract

We report the highest-redshift detection of [O I] 63 $\hat{I}¼$m from a luminous quasar, J2054-0005, at $z=6.04$ based on the Atacama Large Millimeter/sub-millimeter Array Band 9 observations. The [O I] 63 $\hat{I}¼$m line luminosity is $(4.5\pm 1.5)\times 10^9{L}_{\odot }$, corresponding to the [O I] 63 $\hat{I}¼$m-to-far-infrared luminosity ratio of $\approx 6.7\times 10^{-4}$, which is consistent with the value obtained in the local universe. Remarkably, [O I] 63 $\hat{I}¼$m is as bright as [C II] 158 $\hat{I}¼$m, resulting in the [O I]-to-[C II] line luminosity ratio of $1.3\pm 0.5$. Based on a careful comparison of the luminosity ratios of [O I] 63 $\hat{I}¼$m, [C II] 158 $\hat{I}¼$m, and dust continuum emission to models of photo-dissociation regions, we find that J2054-0005 has a gas density log($n_{\mathrm{H}}$/cm${}^{-3}$)$=3.7\pm 0.3$ and an incident far-ultraviolet radiation field of log($G/G_0$)$=3.0\pm 0.1$, showing that [O I] 63 $\hat{I}¼$m serves as an important coolant of the dense and warm gas in J2054-0005. A close examination of the [O I] and [C II] line profiles suggests that the [O I] line may be partially self-absorbed, however deeper observations are needed to verify this conclusion. Regardless, the gas density and incident radiation field are in a broad agreement with the values obtained in nearby star-forming galaxies and objects with [O I] 63 $\hat{I}¼$m observations at $z=1-3$ with the Herschel Space Observatory. These results demonstrate the power of ALMA high-frequency observations targeting [O I] 63 $\hat{I}¼$m to examine the properties of photo-dissociation regions in high-redshift galaxies.

Accepted for publication to PASJ. 5 figures and 3 tables